»Is there a way I can check if my username has already be taken?

I'm wondering if I can do this because I don't want to wait another 30 days before I change my name.

 

You have to go in the server and see if the server asks you to login - means that username is taken.

 

I have premium mc account so basically I have to wait 30 days before changing my name.
Well I will probaly survive 30 days without gc

 

when your name changes, other people can not steal your old name for approximately 35 days

 

So if I change my name back the password for that account will still be working?

 

www.namemc.com

 

I mean if there is a way I cam check if there is my name has already been taken on GUILDCRAFT

 

Go to Guildcraft with the username you want and see if the chat says: "Please log in with /login <password>"
If that message comes up in chat, it means the username is taken. And if the server instead asks you to register with a new password; it means you can use that username to create an account. Hopefully your question's been answered.
~Requesting lock~
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